This invention relates in general to the welding of metal tubes or pipes and, more specifically, to an improved backup mandrel for use in such welding.
A fusion welding system is one current type of system used for butt welding metal structures which are in close fitting contact along a weld line or for repairing defects such as cracks in metal structures. In fusion welding, a welding head is placed on one side of the structure to be welded. Then the head is adapted to heat metal along the weld line to fusion temperature. A welding backup apparatus typically engages the side of the structure opposite to the welding head and is also aligned with the welding location. In addition, the apparatus used must resist significant degradation at the temperatures used to fuse the metal.
In butt welding thin sheets, tubes or other thin structures together, a number of problems may be encountered. Welding such structures without support along the weld line opposite the welding head is generally unsatisfactory. Molten metal from the weld zone tends to drop through, leaving voids along the weld line. Also, the underside of the weld tends to be uneven, with droplet-like projections. In an attempt to overcome some of the problems mentioned above, prior art has shown developments in use of backup devices in the form of tapes. One such example is in Roden et al. U.S. Pat. No. 4,049,183, which described a welding backup tape. Attempts have been made to support weld lines with tape-like support means of various kinds, adhesively bonded along the weld line. The surfaces of some backup tapes are intended to fuse during the welding operation, and may include a surface layer of a flux. This type tape is undesirable in high quality aerospace type applications where the weld must be free of contamination. Furthermore, tape-like supports are not usable on weld joints far removed from either end of a tube assembly. Other commercial tapes, while satisfactory for rough work or where the weld bead is to be finish machined, do not provide adequate support to produce highly uniform, smooth weld beads. In some cases, local failure of the tape from thermal weakening or melting will permit drop through of the fused metal, leaving insufficient metal to form a satisfactory bead, especially where no metal is added during welding.
In order to provide sufficient support, many tapes require a rigid backup plate pressing the tape against the weld underbead. In welding complex structures, such as along tubes, it is inconvenient to attempt to hold the tape in place with a solid member. Since the solid backing member must conform to the surface being welded, a special backing member must be built for each structure design being welded. Other backup tapes or strips may include a metal or solid refractory strip to provide rigid support. These tapes are complex, heavy, difficult to install and will only conform to curved weld surfaces of large radii of curvature.
Therefore, with the above noted problems, there is a continuing need for an improved welding backup apparatus for use in welding metals such as stainless steel or aluminum to high quality standards.